The present invention relates to a two-sided copying method and apparatus for an image recording apparatus of the type having a document feeder and, more particularly, to a two-sided copying method for a digital copier.
Some modern copiers are equipped with an automatic document feeder (ADF) for automatically feeding original documents one by one to a document support in the form of a glass platen and have the capability of producing copies each carrying images on both sides thereof, i.e. two-sided copies. Two-sided copying involves a variety of situations such as one in which the last page of documents which are to be reproduced on both sides of individual papers is odd and one in which it is even. A prior art two-sided copying method heretofore known in various forms is in many cases insufficient to match those situations to ordinary one-sided copying operations. For example, there has been known a two-sided copying method which causes an ADF to feed the first document to a glass platen, then reproduces the document on one side of a paper sheet, transports the resulting one-sided copy to a two-sided tray to store it temporarily, and then refeeds the one-sided copy while driving the ADF to feed the next document to the glass platen, thereby reproducing the subsequent document on the other side of the one-sided copy. A drawback with such prior art method is that the processing rate and therefore the efficiency achievable is extremely low, especially in a so-called one-to-one copy mode available for producing a single two-sided copy. Specifically, the processing rate of a copier for producing a single one-sided copy is usually 60% to 80% of the processing rate associated with a continuous copy mode, while the processing rate available for producing a single two-sided copy by the prior art two-sided copying procedure is as low as about 20% to 30%.
Another two-sided copying method known in the art is such that an ADF sequentially feeds a stack of documents to a glass platen, a document bearing the last page first. This kind of method has a shortcoming in that when the last page is an odd page, the back of the first two-sided copy, i.e., the back of the first page is left blank. Such shortcoming will of course be eliminated if an ADF is caused to sequentially feed documents to a glass platen, the first page of the first document first. Then, however, the resulting copies will be piled up on a copy tray with the last page atop the pile and will therefore require an extra mechanism for turning over the individual paper sheets to put them in order with respect to correct numbering of the pages. With a copier which lacks such a mechanism, therefore, it would be advantageous to feed documents sequentially to a glass platen, the last document bearing the last page first.
Further, a copier with a recycling ADF (RADF) is known which produces two-sided copies by causing the RADF to recycle individual documents. This type of copier adopts a method which drives the RADF to feed odd documents one at a time to a glass platen, reproduces the odd documents on one side of individual paper sheets, stacks the resulting one-sided copies on a two-sided tray, and refeeds the one-sided copies one by one from the two-sided tray while causing the RADF to feed even documents to the glass platen to reproduce them on the other side of the individual one-sided copies. While such a procedure successfully increases the processing rate to a certain degree, it involves the need for repetitively feeding each of the documents. More specifically, even the even documents have to be fed while the copying cycle is repeated with the odd documents, and vice versa. This is likely to bring about a problem of damage to the individual documents and therefore requires the reliability of operation of the RADF to be enhanced accordingly.